1. Field of the Invention
The present invention relates in general to a process for fabricating bipolar junction transistors. In particular, the present invention relates to a process for fabricating bipolar junction transistors having fast switching speed.
2. Technical Background
Conventional semiconductor bipolar junction transistor (BJT) devices are characterized by the limitations in their feasible operation switching speed. At least several causes are responsible for this limited switching speed. One of them is the excessive parasitic capacitance found in the junction of the emitter region of the BJT device, and the other is the long distance that separates the emitter region and the contact region of the collector region thereof.
FIG. 1 of the accompanying drawing of the present invention schematically depicts the cross-sectional view of a conventional BJT device, which is utilized herein for the purpose of the description of the present invention.
Refer to FIG. 1. In a typical prior art process, the fabrication of a BJT device begins with an N-type semiconductor silicon substrate 1. The substrate 1 itself plays the role of the collector region for the BJT device being fabricated. A P-well 10 is then formed in the N-type substrate 1 that can be utilized as the base region of the BJT. Within this basic structural configuration, N-type emitter 12, P+-type contact region 14 for the P-type base region 10, and the N+-type contact region 16 for the N-type collector 1 are formed as is seen in the drawing of FIG. 1.
As is depicted in the drawing, which, although not drawn to the exact scale, does schematically show the structural configuration of the BJT device fabricated in the conventional process, it can be found that the surface area of the emitter region 12 is relatively large resulting in the presence of parasitic capacitance between its junction with the P-type base region 10. On the other hand, the distance from the emitter region 12 to the contact region 16 of the collector region 1 is excessively long, this places a restriction on the possible device switching speed that may be attained. Further, this long distance also is a factor that restricts the possible amount of device gain level.